At present, thermal transfer recording processes which employ heating units such as thermal heads, etc. are widely practiced in recording devices such as printers as computer terminals, word processors, facsimile machines, copying machines, etc. Those thermal transfer recording processes are classified into a thermal transfer recording process which employs heat-resistive paper and a thermal transfer recording process which employs thermal transfer ink ribbon.
The thermal transfer recording process which employs thermal transfer ink ribbon is referred to as thermal fusion and transfer recording process. According to this process, an ink layer is disposed on a base in the shape of a tape, and a transfer medium (printing medium) such as paper is held in intimate contact with the ink layer. A thermal head is applied to the reverse side of the base to heat the base, fusing the ink layer with heat and transferring the ink to the transfer medium. Heretofore, the ink layer comprises a colorant, a filler, etc. which are rendered formable by a binding material (hereinafter referred to as a binder) that mainly comprises wax, and is deposited on the base to a thickness of several .mu.m.
In recent years, transferred images that are printed using such a thermal transfer ink ribbon have been required to be highly sharp in appearance. Since sharper transferred images can be obtained as the ink layer has a greater ability to be separated sharply from the base, it has been attempted to add fine particles of a heat-setting resin such as silicone resin or the like to the ink layer of the thermal transfer ink ribbon whose binder main comprises wax for thereby improving the ability of the ink layer to be separated sharply from the base, and such attempts have proven somewhat effective (Japanese laid-open patent publication No. 3-239589).
However, with the thermal transfer ink ribbon whose binder mainly comprises wax, images transferred to a transfer medium by the thermal transfer process are poor in heat resistance and wear resistance, and hence have insufficient durability.
It has been proposed to use a thermally fusible resin, i.e., a thermoplastic resin, instead of wax, as a main constituent of the binder. While the use of a thermoplastic resin is effective to improve the heat resistance and wear resistance of transferred images, however, the ability of the ink layer to be sharply transferred from the base is reduced. In a boundary between an area of the ink layer which has been heated by the thermal head and an area of the ink layer which has not been heated by the thermal head, the ink layer may not sharply be separated from the base, failing to produce a sharp transferred image.
In an effort to solve the above problem with respect to the ability of the ink layer to be sharply separated from the base, the inventors of the present invention have proposed a thermal transfer ink ribbon as disclosed in Japanese laid-open patent publication No. 5-286272.
The disclosed thermal transfer ink ribbon has an ink layer comprising a colorant, a thermoplastic resin, and particles of a fluorine-containing resin or a silicone resin, and allows the ink layer to be separated sharply to produce sharp transferred images while maintaining desired heat resistance and wear resistance thereof.
If the transfer medium constitutes a label, then images transferred using the above thermal transfer ink ribbon may be sharp or not sharp depending on the type of the label, and hence have no constant printing quality. There are various factors which make transferred images not sharp, and it has been desired to analyze the factors and establish measures to eliminate those factors.